Treatment of Waterborne Wastes
The treatment of waterborne wastes can be broadly divided into two stages, viz., primary treatment and secondary treatment. In addition, a third stage or tertiary treatment process may be required in some circumstances to protect the uses made of the receiving waters. Primary treatment is concerned with the separation of as much as possible of the solids carried in suspension in the sewage, and their subsequent treatment and disposal. Solid matter is removed in preliminary treatment units, such as screens and grit traps, and by settlement in tanks which provide quiescent conditions for the removal of the solids by gravity settling. Secondary treatment involves the biological stabilisation of the liquid effluent, including the remaining suspended solids and the dissolved solids, from the primary treatment processes. Examples of secondary treatment are the trickling filter and activated sludge processes. The tertiary treatment or “polishing” of an effluent to remove any residual polluting matter or bacteriological contamination may be effected by the use of sand filtration or oxidation ponds, among other processes. In some cases two or more of these stages of treatment are carried out in a single treatment unit such as an oxidation pond or a Pasveer oxidation channel.
In 1961 Government approved the payment of subsidies to local authorities towards the cost of providing sewage treatment and disposal facilities for towns with populations of less than 20,000. Because the cost per head of population served by such facilities is generally higher in the case of small communities than for cities, the subsidies are available on a graduated scale, the highest subsidy (50 per cent) being payable to communities with a population of less than 1,000.